The present invention relates to a magnetic disk device for rotating a magnetic disk to record and reproduce information through a magnetic head, and particularly relates to the improvement that the quantity of the deviation between the magnetic head and the data track of the magnetic disk is reduced.
FIG. 4 shows a plan view of a conventional magnetic disk device disclosed in Japanese Utility Model Application (OPI) No. 76569/86 (the term "OPI" as used herein means an "unexamined published application"). Shown at 1 in FIG. 4 is a magnetic disk fitted on a hub 2 at the end of the shaft of a spindle motor (not shown in the drawing) so as to be rotated. A magnetic head 3 records and reproduces information onto and from the magnetic disk 1 and is attached through a support spring 3a to a swing arm 4 rotatably supported on a pivot 5. One end of a band 6 is coupled to the swing arm 4. A capstan 7 is secured to the shaft 8 of a step motor not shown in the drawing. The band 6 is wound on the capstan 7 so as to be moved to turn the swing arm 4. The spindle motor for the hub 2, the pivot 5 and the step motor for the capstan 7 are attached to the upper plate of a box-shaped base 9 whose bottom is open.
A data track is circularly provided on the top of the magnetic disk 1 which is rotated at high speed. The capstan 7 is rotated so that the swing arm 4 is turned through the band 6. After the magnetic head 3 is positioned over a desired part of the data track, the rotation of the capstan is stopped.
Such a conventional magnetic disk device is usually required to operate properly despite an ambient temperature change of 40.degree. to 50.degree. C. However, it is likely that the components of the device are thermally deformed due to thermal expansion or contraction under the ambient temperature change. In that case, the magnetic head 3 of the device and the data track on the magnetic disk 1 thereof go out of a prescribed positional relation to each other, namely, the magnetic head and the data track deviate from each other so that information cannot be properly recorded and reproduced onto and from the magnetic disk.
The material of each component of the conventional magnetic disk device is usually as follows:
The magnetic disk 1 is made of pure aluminum. The magnetic head support spring 3a is made of a stainless steel. The band 6 is made of a stainless steel. The swing arm 4 is made of an aluminum alloy. The base 9 is made of an aluminum alloy. The capstan 7 is made of a stainless steel or an aluminum alloy.
Since the materials of the components of the magnetic disk device differ from each other as mentioned above, the quantities of thermal deformation of the components also differ from each other so that the magnetic head and the data track deviate from each other. The cause of the deviation is that the coefficients of thermal expansion of the stainless steel, the aluminum alloy and the pure aluminum as the materials of the components differ from each other in the order of (stainless steel)&lt;(aluminum alloy)&lt;(pure aluminum). It is practically very difficult to change the materials of the magnetic disk 1, the magnetic head support springs 3a and the band 6. For that reason, the base 9 is conventionally made of the aluminum alloy of 14 to 16% by weight in silicon content to decrease the coefficient of thermal expansion of the base so that the deviation between the magnetic head 3 and the data track is reduced.
However, since the silicon content of the aluminum alloy of the base 9 is 14 to 16% by weight, which is larger than that (12% or less by weight) of an ordinary aluminum alloy, the cost of the material of the base is high and the hardness of the material is so high as to deteriorate the working property of the material. For that reason, there is a problem that the material cost and the working cost of the base 9, which has the largest volume in all components of the magnetic disk device and has a large worked surface area, are very expensive.
Further, since the quantity of the deviation between the magnetic head and the data track depends on the combination of the materials of the components in the magnetic disk device, there is another problem that the quantity of the deviation is not necessarily reduced even if the base 9 is made of a material having a small coefficient of thermal expansion.